Abstract Book

S961

ESTRO 37

treatment plans of the patients treated within the Fist- in-Man (FIM) study. Material and Methods Patients with painful lumbar spine bone metastases were treated within the FIM study. The vertebrae and the spinal cord are delineated using the pre-treatment CT and MRI. For the actual treatment the delineations were propagated to the online MRI, also a pseudo CT was created using deformable registration. Both were used for on-line treatment planning. Step-and-shoot IMRT plans were generated using the Monaco planning system (version 5.19, Elekta AB, Sweden) which includes the effects of the magnetic field on the dose distributions. The beam model was commissioned for the 7 MV MRI Linac. All plans were automatically created using the same template, which includes 3 or 5 beams depending on the location of the vertebrae. The prescribed dose to the PTV was 8 Gy in a single fraction. The QA result presented are for: 1) test plans created prior to clinical introduction using patient data of 20 existing patients and 2) pre-treatment and online generated plans of the 4 patients in the FIM study. All plans were recalculated on a polystyrene slab phantom (30x30x10 cm3) and on the ArcCheck-MR phantom (Sun Nuclear GmbH, Germany). The ionization chamber measurements were performed at the center of the slab phantom using an IBA CC04 ionization chamber (IBA dosimetry GmbH, Germany). Film measurements were also performed by placing radiochromic EBT3 films in the center of the phantom (isoc) in the coronal plane. To evaluate the absolute dose distribution, the film was normalized to the ionization chamber dose at isoc. For both the film and ArcCheck dose distributions, a gamma analysis was performed using a 3% and 3 mm criteria. Results The QA results of the test plans and the plans created for the FIM study are very similar as shown in the table. The average difference between the calculated dose and dose measured with the ionization chamber was small (< 0.5%). The gamma analyses of the film measurements also showed that the measured absolute dose is in good agreement with calculated dose distribution with gamma pass rates better than 98 %. The gamma analysis of the measured dose obtained with the ArcCheck-MR also resulted in high gamma pass rates (>98 %). Conclusion The results of the QA test performed with various detectors and phantoms for the IMRT plans generated for the FIM plans show that the calculated dose and the measured dose distribution are in good agreement. EP-1791 Evaluation of a water equivalent EPID model for flattening filter free (FFF) beam transit dosimetry S. Deshpande 1,2,3 , S. Blake 3,4 , L. Holloway 1,2,3,4 , P. Vial 1,3,4 1 Liverpool and Macarthur Cancer Therapy Centre, Department of Medical Physics, Sydney, Australia 2 University of New South Wales, South Western Sydney Clinical School, Sydney, Australia 3 Ingham Institute, Applied medical research, Sydney, Australia 4 University of Sydney, Institute of Medical Physics- School of Physics, Sydney, Australia Purpose or Objective Our research group has developed and benchmarked water-equivalent electronic portal imaging devices (WE- EPID) for dosimetry of conventional flattening filter (FF)

. Conclusion The gamma evaluation was able to detect both geometric and dosimetric errors. Thus, the in-house developed EPID-based 3D dose reconstruction algorithm shows adequate sensitivity to errors for plan specific pre- treatment QA of advanced VMAT delivery. Moreover, the gamma evaluation could support the interpretation of the error causes. EP-1790 Patient specific quality assurance on the MRI Linac for the First-in-Man study B. Van Asselen 1 , S.L. Hackett 1 , J.W.H. Wolthaus 1 , J.H.W. De Vries 1 , I.M. Jürgenliemk-Schulz 1 , J.J.W. Lagendijk 1 , B.W. Raaymakers 1 1 UMC Utrecht, Department of Radiation Oncology, Utrecht, The Netherlands Purpose or Objective This work describes the tests for patient-specific quality assurance (QA) performed during the clinical introduction of the MRI linac (Elekta AB, Sweden) and used for the